Process for the adjustment of the glass temperature in a float glass lehr and float glass lehr in order to bring this process into operation

ABSTRACT

A method and an apparatus for the heat treatment of a float glass ribbon in which a tunnel lehr is provided with arrays of parallel heat-exchange tubes above and below the path of the ribbon and for each of the longitudinally divided zones of these arrays, the tubes are grouped and provided with respective heaters at the upstream end of the zone while all of the tubes at the zone are connected to a flow at the downstream end for drawing the heated air through the tubes. The temperature of the glass ribbon is measured at the upstream end and the throughflow of the individual groups is controlled selectively to ensure a homogeneous temperature distribution in the glass ribbon.

FIELD OF THE INVENTION

This invention relates to a process for the adjustment of the glasstemperature in a float glass lehr with exchanging tubes. The inventionalso relates to a float glass lehr operated in accordance with thisprocess.

BACKGROUND OF THE INVENTION

Flat glass, manufactured in a continuous way, is conveyed through a heatinsulated tunnel, called a float glass lehr, which is provided withheating means and with cooling means and in which glass is cooled andannealed.

In conventional float glass lehr, the cooling means are composed of aseries of exchanging tubes arranged in two horizontal rows inside thetunnel, above and below the glass ribbon. These exchanging tubes arearranged in transverse batteries around in which air flows, initially atthe ambient temperature, in the opposite direction to the glass traveldirection. The heating means are electrical means and consist of bare orshielded wire heaters located between the exchanging tubes and theglass. The glass cooling can be modified by acting on the air flow rate;the heating can also be modified by acting on the power emitted by thewire heaters.

This glass temperature control process has a number of serious drawbacksespecially when there is a ribbon of transparent glass and/or of glasswith a highly reflecting coating on one face.

As a matter of fact, glass is a poor conductor of heat and during itscooling a noticeable temperature gradient appears between the surfaceand the middle part of the glass ribbon. If the glass is still plastic,the stresses created by the temperature gradient produce a deformationof the glass ribbon. If the glass is sufficiently cooled and perfectlyelastic the temperature gradient creates internal stresses in the glassribbon which remain since the ribbon is no longer plastic.

Experience shows that, in order to reduce as much as possible the effectof the temperature gradient, it is advisable to achieve an even glasscooling over the whole glass lehr length, i.e. the glass temperatureshould decrease approx. in a linear way. Now, it should be noted thatwhen cold air is introduced at the inlet end of the exchanging tubes,the cooling is not at all even; glass is cooled much more rapidly at theoutlet than at the inlet of the lehr zone concerned so that internalstresses are induced in glass in the middle of the annealing range.

On the other hand, glass is cooled more rapidly on the longitudinaledges than in the middle part. If, in order to remedy this transversetemperature gradient, the heating means located close to thelongitudinal edges of the float glass lehr are put into operation, theseheating means radiate on the adjacent cooling means; owing to thisinteraction it is difficult to achieve an optimum and uniformtemperature over the whole width of the glass ribbon.

It is very difficult to remedy these drawbacks as the cooling means havea very long response time whereas the electrical heating means have arelatively short response time and can cause glass overheating when theyare brought into operation.

OBJECT OF THE INVENTION

The object of this invention is to provide a process for the adjustmentof the glass temperature in a float glass lehr which remedies thesedrawbacks.

SUMMARY OF THE INVENTION

According to the invention, the temperature is adjusted by means of theexchanging tubes only, heated air being admitted at the inlet of thesetubes.

If relatively hot air enters the tubes, it is possible to achieve aheat-up of the float glass lehr or to keep it at temperature withoutglass. If cooler air, at a temperature below the glass temperature,enters, it is possible to cool the glass during continuous productionand to achieve an even glass cooling.

The air flowing in the tubes can circulate in an open circuit or in aclosed circuit.

This invention also relates to a float glass lehr operated by thisprocess.

According to the invention there is provided a float glass lehr thespecial feature of which is that it comprises means for heating the airintroduced into the exchanging tubes.

These heating means can be either electrical means or burners or canconsist of indirect exchangers with heat exchanging medium.

If the air which flows in the tubes circulates in a closed circuit, thefloat glass lehr can also include means for cooling the recirculatedair. These means can, for example, consist of a cold air introductionsystem or of indirect exchangers with cooling fluid.

BRIEF DESCRIPTION OF THE DRAWING In the Drawing:

FIG. 1 is a schematic perspective view of a zone of a first embodimentof float glass lehr according to the invention;

FIG. 2 is a longitudinal section through a part of this zone; and

FIG. 3 is a view similar to FIG. 1 of a second embodiment of float glasslehr according to the invention.

SPECIFIC DESCRIPTION

The first embodiment of a float glass lehr, according to the invention,shown in FIGS. 1 and 2 comprises a heat insulated tunnel 1 that isprovided with rollers 2 supporting a glass ribbon 3 travelling in thedirection of arrow f1.

The lehr is divided into successive zones over its length. In each zone,it comprises two horizontal rows of longitudinal exchanging tubes 4a and4b, one arranged above and the other below the ribbon 3. Each row ofmutually parallel coplanar transversely spaced tubes is dividedtransversely into batteries which there are five in number in the formof float glass lehr shown herein, each battery comprising a plurality oftubes.

The inlet of each battery of tubes, located at the downstream side ofthe zone concerned (with respect to the direction of movement of theglass ribbon) is connected to a flow control valve 5 and to heatingmeans 6, for example with burners in which cold air is admitted in 7.The outlet of the tubes is connected to a collector 8 at the suctionside of a fan 9 supplying a secondary heat recovery system.

Air thus circulates in the opposite direction to the glass traveldirection.

A sensor 10 measures the glass temperature at the zone outlet side andacts by means of control systems (not shown) on the valves 5 and on theheating means 6.

By acting on the valves and on the heating means of the differentbatteries it is possible, for example, to heat the external edges of thesheet 3 and to cool the middle zone.

In its major principles, the form of float glass lehr shown in FIG. 3 issimilar to that shown in FIGS. 1 and 2 but the air circulates in aclosed circuit. The outlets of the fans 9 are connected by a pipe to acollector 12 supplying the various heating means 6. A hot air exhaustpiping 14 and a cold air inlet piping 13 are connected on both sides ofthe fan. The air flow and the cooling capacity variation are thenobtained by varying the speed of the fans 9, the cold air flow admittedthrough the piping 13 and the hot air flow exhausted through the piping14. The hot air exhaust is connected to a secondary heat recoverysystem. .[.What we claim is.].

.Iadd.I claim: .Iaddend.
 1. In a method of operating a float glass lehrin which a float glass ribbon is passed on rollers through a lehr tunneland is subjected to heating and cooling therein, and wherein arrays oftubes for heat exchange are provided above and below the glass ribbon,said tubes extending parallel to the direction of movement of the glassribbon through the lehr, the improvement which comprises:(a) formingeach array in sets of mutually parallel transversely spaced coplanartubes in successive zones along said path and grouping a plurality oftubes of each array for the respective zone in groups whereby for eacharray and each zone a respective plurality of such groups is disposedtransversely of the direction of movement of the glass ribbon throughthe lehr; (b) heating air separately for each of said groups andadmitting the heated air to the tubes of each group at an .[.upstream.]..Iadd.downstream .Iaddend.end of the respective zone with respect tosaid direction of movement of the glass ribbon through each lehr; (c)drawing heated air from the .[.downstream.]. .Iadd.upstream.Iaddend.ends of the tubes of each zone; and (d) measuring thetemperature of the glass ribbon at the upstream end of each zone andselectively controlling the rates of flow through the respective groupsof tubes in response to the measured temperature, thereby maintainingtemperature homogeneity of the glass ribbon.
 2. The improvement definedin claim 1 wherein the heated air is withdrawn from all of the tubes ofeach array in each zone in common and at least part of withdrawn heatedair is recirculated to the individual groups of the respective array andzone.
 3. A float glass lehr for the heat treatment of a float glassribbon, said lehr comprising:an elongated lehr tunnel; a multiplicity ofrollers spaced apart longitudinally of said tunnel but extendingtransversely therein and forming a path along which said ribbon isdisplaced on said rollers; a lower array of longitudinally extending,mutually parallel coplanar heat-exchanger tubes disposed below saidpath, and an upper array of mutually parallel coplanar transverselyspaced heat-exhange tubes disposed above said path within said tunneleach of said arrays being subdivided longitudinally into respectivezones, each zone having an upstream side and a downstream side withrespect to the direction of travel of said ribbon along said path; meansconnecting a plurality of tubes of each array and each zone in arespective group whereby, for each zone, each of said arrays comprises aplurality of such groups in succession transversely of said direction; arespective air heater communicating with the tubes of each group at anupstream end of the respective zone whereby heated air flows throughsaid tubes in a direction opposite the direction of movement of saidglass ribbon along said path; means at the downstream end of each ofsaid zones for drawing air from said tubes; and means at the upstreamend of each zone for measuring the temperature of the glass ribbon andcontrolling selectively the heating effect contributed by each group tosaid glass ribbon.
 4. The lehr defined in claim 3, further comprisingmeans for recirculating heated air from the downstream end of each zoneto the respective heaters of the group at the upstream end of therespective zone.
 5. The lehr defined in claim 4, further comprisingmeans for cooling the recirculated air.
 6. In a method of operating afloat glass lehr in which a float glass ribbon is passed on rollersthrough a lehr tunnel and wherein ducts for heat-exchange with the floatglass ribbon are provided above and below the glass ribbon and extendparallel to the direction of movement of the glass ribbon through thelehr, the improvement which comprises:(a) generating hot air at arelatively downstream location along said ducts with respect to thedirection of movement of the glass ribbon through the lehr; (b) passingsaid hot air through said ducts from said downstream location in adirection opposite a direction of movement of the glass ribbon throughthe lehr, thereby heating said ducts to a temperature below that of theglass and thereby cooling the glass ribbon; and (c) collecting air fromsaid ducts at a relatively upstream location thereof with respect to thedirection of movement of the glass ribbon through the .[.layer.]..Iadd.lehr. .Iaddend.
 7. The method defined in claim 6 furthercomprising recirculating hot air collected from said upstream locationto said downstream location, said method further comprising the step ofadmitting fresh air to the recirculated air.